Method for providing void free layer for semiconductor assemblies

ABSTRACT

A method of providing a substantially void free layer for one or more flip chip assemblies, or one or more microelectronic components, utilizing a curable encapsulant. Also disclosed is a method of injecting an encapsulant into an assembly and a method of treating a microelectronic component to form a void free layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of application Ser. No. 09/188,599filed Nov. 9, 1998, now U.S. Pat. No. 6,107,123, which is a divisionalapplication of application Ser. No. 08/610,610 filed Mar. 7, 1996, nowU.S. Pat. No. 5,834,339.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor chippackaging.

BACKGROUND OF THE INVENTION

In the construction of semiconductor chip assemblies, it has been founddesirable to interpose encapsulating material between and/or aroundelements of the semiconductor assemblies in an effort to reduce and/orredistribute the strain and strain on the connections between thesemiconductor chip and a supporting circuitized substrate duringoperation of the chip, and to seal the elements against corrosion.

Ball grid array (“BGA”) packaged and chip scale packaged (“CSP”)semiconductor chips and flip chip attachment solutions are connected toexternal circuitry through contacts on a surface of the chip. To savearea on a supporting substrate, such as a printed wiring board (“PWB”),these chips are directly connected/soldered to the substrates and fromthere connected to external circuitry on other parts of the substrate.The chip contacts are either disposed in regular grid array patterns,substantially covering the face surface of the chip (commonly referredto as an “area array”) or in elongated rows extending parallel to andadjacent each edge of the chip front surface. Many of the techniques forattachment run into problems because of the thermal expansion mismatchbetween the material the chip is composed of and the material thesupporting circuitized substrate is made of, such as a PWB. In otherwords, when the chip is in operation, the chip heats up and also heatsits supporting substrate thereby causing both the chip and the substrateto expand. When the heat is removed, the chip and substrate bothcontract. This heating and cooling process is referred to as “thermalcycling”. Since the heat is being generated in the chip, the chip willheat up more quickly and will typically get hotter than its supportingsubstrate. The materials comprising both the chip and the substrate haveinherent expansion and contraction rates, referred to as theircoefficients of thermal expansion (“CTE”), which causes them to expandand contract at different rates and in different degrees when subjectedto the same thermal conditions. This thermal expansion mismatch betweenthe chips and the substrate places considerable mechanical stress andstrain on the connections between the chip contacts and correspondingbond pads on the substrate.

BGA and CSP technology refers to a large range of semiconductor packageswhich use interconnection processes such as wirebonding, beam lead, tapeautomated bonding (“TAB”) or the like as an intermediate connection stepto interconnect the chip contacts to the exposed package terminals. Thisresults in a device which can be tested prior to mechanical attachmentto the bond pads on supporting substrate. The BGA or CSP packaged chipsare then typically interconnected with their supporting substrates usingstandard tin-lead solder connections. In most such packaged devices, themechanical stress/strain due to thermal cycling is almost completelyplaced on the solder connections between the chip and the substrate.However, solder was never intended to undergo such forces and commonlyundergoes significant elastic solder deformation causing the solder tocrack due to fatigue brought on by the thermal cycling. When the solderconnections have smaller diameters, thermal cycling has an even moreprofound fatiguing affect on the solder. This has driven efforts in thepackaging art to modify the solder and other elements of the packages sothat they may better withstand the thermal cycling forces.

As the features of semiconductor chip packages continue to be reduced insize, as in the case of CSPs, the number of chips packed into a givenarea will be greater and thus the heat dissipated by the each of thesechips will have a greater effect on the thermal mismatch problem.Further, the solder cracking problem is exacerbated when more than onesemiconductor chip is mounted in a package, such as in a multichipmodule. As more chips are packaged together, more heat will bedissipated by each package which, in turn, means the interconnectionsbetween a package and its supporting substrate will encounter greatermechanical stress due to thermal cycling. Further, as more chips areintegrated into multichip modules, each package requires additionalinterconnections thereby increasing the overall rigidity of theconnection between the module and its supporting substrate.

Certain designs have reduced solder connection fatigue by redistributingthe thermal cycling stress into a portion of the chip package itself. Anexample of such a design is shown in U.S. Pat. Nos. 5,148,265 and5,148,266, the disclosure of which is incorporated herein by reference.One disclosed embodiment of these patents shows the use of a chipcarrier in combination with a compliant layer to reduce the CTE mismatchproblems. Typically, the compliant layer includes an elastomeric layerwhich, in the finished package, is disposed between the chip carrier andthe face surface of the chip. The compliant layer provides resiliency tothe individual terminals, allowing each terminal to move in relation toits electrically connected chip contact to accommodate CTE mismatch asnecessary during testing, final assembly and thermal cycling of thedevice.

In some arrangements used heretofore, the compliant layer was formed bystenciling a thermoset resin onto the chip carrier and then curing theresin. Next, additional resin was applied to the exposed surface of thecured layer, this additional resin was partially cured, and theresulting tacky adhesive surface was used to bond the elastomeric layerto the chip and chip carrier. Once attached, the entire structure washeated and fully cured. Although this process is effective, furtherimprovement would be desirable. The ambient gas can be occasionallyentrapped when the chip carrier and die are affixed to the compliantlayer. The entrapped gas can create voids and gas bubbles in theencapsulation of the surface of the die by the encapsulation material.These voids/bubbles allow moisture and other contamination to come intodirect contact with the surface of the die. Accordingly, care must betaken to prevent such entrapment. This adds to the expense of theprocess.

In the flip-chip mounting technique, the contact bearing face surface ofthe chip opposes a bond pad bearing supporting substrate. Each contacton the device is joined by a solder connection to a corresponding bondpad on the supporting substrate, as by positioning solder balls on thesubstrate or device, juxtaposing the device with the substrate in thefront-face-down orientation and momentarily reflowing the solder. Theflip-chip technique yields a compact assembly, which occupies an area ofthe substrate no larger than the area of the chip itself. However,flip-chip assemblies suffer from significant problems when encounteringthermal cycling stress because the sole thermal cycling stress bearingelements are the solder connections, as described above in relation tothe BGA and CSP packages. In the case of flip chip devices, there is nopackage to redistribute the thermal cycling stress. Because of this,significant work has been done in the art to make the flip chip solderconnections more reliable. However, to keep the chip's standoff from thesubstrate to a minimum, the solder connections have a typical diameterof between about five and eight thousandths of an inch (“mils”), toosmall to provide much real mechanical compliance. In an attempt to solvethis problem, a curable liquid underfill is flowed between the chip andits attached substrate, enclosing the solder connections. The underfillis then cured into a rigid form which has a CTE that is closely matchedto the solder material. The aim of the underfill is to reduce the stresscaused by CTE mismatch by redistributing the stress more uniformly overthe entire surface of the chip, supporting substrate and solder balls.Examples of the use of underfill materials may be found in U.S. Pat.Nos. 5,120,678, 5,194,930, 5,203,076 and 5,249,101. All of these priorart solutions are aimed at reducing the shear stress endured by theinterconnections caused by thermal cycling. However, each of thesesolutions also encounters significant problems such as insufficientcompliancy, voids and process cost. Most significant among these costsis reducing the voiding problem which occurs when the underfill flowsbetween the chip and the substrate and traps gas therebetween. If thisgas is not removed, it will typically quickly expand during a heatingcycle of the chip. The force associated with the expanding gas can causethe solder connections to crack or otherwise become unreliable. Yet,presently, the underfill process involves a very costly andtime-consuming process of allowing the underfill to flow very slowlybetween the chip and the substrate to try to avoid voids. After theunderfill has flowed completely between the chip and the substrate, theassembly will then be subjected to one or more vacuuming steps in afurther attempt to remove any voids in the underfill material.

Despite these and other efforts in the art, still further improvementsin interconnection technology would be desirable.

SUMMARY OF THE INVENTION

The present invention provides a method of eliminating voids and gasbubbles within the encapsulation used in attaching and packagingmicroelectronic devices which solves the aforementioned problems in theart. The present invention further provides an effective method offilling cavities and channels during encapsulation of a plurality ofsemiconductor chips formed on a semiconductor wafer.

In one embodiment, the method includes providing a substantially voidand bubble free underfill for a semiconductor wafer having a pluralityof flip chip assemblies. Typically, a flip chip device is electricallyand mechanically attached to a circuitized substrate, such as a PWB, bya plurality of conductive members, which are most typically a pluralityof solder balls. These solder balls provide an electrical path from eachchip contact to a respective bond pad on the substrate. The solder ballsfurther provide a gap or stand-off between the wafer and hence each flipchip device and its substrate. This gap is then sealed on all sides ofthe flip chip device with a curable liquid encapsulant so that either avoid (vacuum) or an area containing a first gas is thereby created. Anisostatic or hydrostatic pressure is then applied to the semiconductorwafer assembly causing the encapsulant to flow into the gap and aroundthe solder balls. An energy is applied to cure the encapsulant once thevoid/bubble has been completely removed thereby ensuring that new voidsand/or bubbles do not re-occur between the flip chip device and thesubstrate. Typically, heat and/or ultra-violet radiation are used as theapplied energy.

In another embodiment of the present invention, the method includescreating a substantially void/bubble free interposer layer between asemiconductor wafer having a plurality of microelectronic components anda sheet-like substrate. According to this method, an interposer layer isinjected into a gap between each of the microelectronic components andthe substrate such that any voids or gas bubbles are sealed within thegap. Isostatic or hydrostatic pressure is then applied to the entiresemiconductor wafer assembly which causes the total volume for thevoids/bubbles to be reduced to the point where they are substantiallyeliminated from the interposer layer. A further step of applying energy,such as heat, is employed to cure the interposer layer thereby ensuringthat future void/bubble do not occur.

The injecting step may include providing a curable, liquid encapsulantat each edge of the gap between the microelectronic components and thesubstrate prior to the step of applying pressure, effectively sealingthe space between the component and the substrate. When the pressure isthen applied, it causes the sealed volume to be reduced thereby allowingthe encapsulant to flow into the gap and form a substantiallyvoid/bubble free interposer layer.

A still further embodiment of the present invention includes a method oftreating an interposer layer for a semiconductor wafer assembly toprovide a substantially void/bubble free interposer layer. An interposerlayer is first disposed between a semiconductor wafer having a pluralityof semiconductor chips and a sheet-like substrate such that any voidswithin or at the boundaries of the interposer are sealed within theassembly. An isostatic or hydrostatic pressure is then applied to theassembly thereby reducing the volume of the voids/bubbles andsubstantially eliminating them from the interposer layer.

The foregoing and other objects and advantages of the present inventionwill be better understood from the following Detailed Description of aPreferred Embodiment, taken together with the attached Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a top plan view of a flip chip device attached to acircuitized substrate having encapsulant sealing the edges therebetween,according to the present invention.

FIG. 1B shows a fragmentary side view of the elements shown in FIG. 1A,according to the present invention.

FIG. 1C shows a bottom plan view of the elements shown in FIG. 1A withthe substrate removed from view, according to the present invention.

FIG. 1D shows a bottom plan view of the elements shown in FIG. 1A withthe substrate removed from view and after full encapsulation, accordingto the present invention.

FIG. 2A shows a top plan view of a semiconductor package assembly havingan encapsulation sealing the edges of a gap between a sheet-likesubstrate and a semiconductor chip, according to the present invention.

FIG. 2B shows a fragmentary side view of the elements shown in FIG. 2A,according to the present invention.

FIG. 2C shows a fragmentary side view of the elements shown in FIG. 2Aafter full encapsulation, according to the present invention.

FIG. 2D shows a perspective view of a plurality of chips attached to asubstrate which is connected to a frame to support the substrate,according to the present invention.

FIG. 2E shows a side view of a nozzle for depositing the encapsulantaround the edges of the gap between the chips and the substrate in FIG.2D so as to seal the voids.

FIG. 2F shows a perspective view of the a plurality of the frames showin FIG. 2D are placed within a single carrier.

FIGS. 2G and 2H show cross sectional side views of two packaged devices,according to the present invention.

FIG. 2I shows a face view of the chip package described in FIGS. 2G and2H, according to the present invention.

FIG. 2J shows a cross sectional view of a center bonded package as thevoid is being diffused into the encapsulation material, according to thepresent invention.

FIG. 2K shows cut-away cross section A from FIG. 2J depicting thecollapsing of the void in response to isostatic pressure, according tothe present invention.

FIG. 2L shows a cut-away cross sectional view of a fan-in/fan-outembodiment, according to the present invention.

FIGS. 2M-1 through 2M-7 show the process of creating a void free layerby injection using the center bonded package shown in FIG. 2J, accordingto the present invention.

FIGS. 2N-1 through 2N-7 show the process of creating a void free layerby injection using the center bonded package having a peelable compliantlayer, according to the present invention.

FIG. 3A shows a top plan view of a semiconductor package assembly havingan interposer layer disposed between a sheet-like substrate and asemiconductor chip, according to the present invention.

FIGS. 3B and 3C show fragmentary side views of the elements shown inFIG. 3A, according to the present invention.

FIG. 3D shows a fragmentary side view of the elements shown in FIG. 3Aafter the voids/bubbles have been removed, according to the presentinvention.

FIG. 3E shows a fragmentary side view of the elements shown in FIG. 3Aafter the package's leads have been encapsulated and the package isconnected to a circuitized substrate, according to the presentinvention.

FIG. 4A shows a perspective illustration of a semiconductor wafer havinga plurality of chips for encapsulation in accordance with one embodimentof the present invention.

FIG. 4B shows a perspective illustration of a semiconductor wafer havinga plurality of chips for encapsulation in accordance with anotherembodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The present invention is directed toward the pressure injection ofencapsulants into cavities and gaps in microelectronic structures andthe simultaneous removal of voids and gas bubbles within encapsulantsand adhesive/chip attach layers. As described below, the presentinventive methods may be used as a method for underfilling attached flipchip devices, for injecting encapsulant and for removing voids and gasbubbles within encapsulants. As will be appreciated by one skilled inthe art, each of the embodiments described below could and preferablywould be performed on more than one assembly at a time to facilitate themass production of finished parts.

I. Flip Chip Underfill Encapsulation

FIGS. 1A-D show a preferred method of providing a substantially gasbubble/void free underfill for a flip chip assembly, according to thepresent invention. FIG. 1A shows top fragmentary view of a flip chiptype assembly including a semiconductor chip 100, a supporting substrate120 (such as a PWB), a plurality of electrical connections 110interconnecting corresponding chip contacts (not shown) and bond pads(not shown) on the substrate 120. The solder connections bothelectrically and mechanically interconnect the chip contacts to the bondpads and, as best shown in FIG. 1B, provide a “stand-off” or gap betweenthe chip 100 and the substrate 120. Although many metals, conductivecomposites and alloys would be suitable to act as the electricalconnections 110, the electrical connections 110 are typically comprisedof an alloy of tin and lead, referred to generically in thesemiconductor packaging industry as “solder balls”. FIG. 1C shows abottom view of the assembly shown in FIG. 1A with the substrate 120removed from view.

As shown in FIGS. 1A-B and the side fragmentary view of FIG. 1B, acurable, liquid encapsulant material 130 is deposited at each of theedges of the chip 100 so as to seal the gap between the chip 100 and thesubstrate 120 creating a sealed void or gas bubble 140 therebetween(depending on whether the surrounding environment has been evacuatedprior to the sealing step). For the sake of simplicity, this void/bubble140 will generically be referred to in this specification and in theclaims as a “void”. Typically, in flip chip embodiments, the encapsulantis comprised of a curable thermoset resin material. Since it is desiredin a flip chip mounted device to have the encapsulant rigid after it iscured, an example of a suitable resin would include an epoxy resin, suchas Hysol® epoxy sold by Dexter Corporation of Pittsburg, Pa.

Isostatic pressure is then applied to the outside of the assembly. Thisstep of applying more pressure on the assembly after the void 140 hasbeen sealed will cause a pressure differential between the compressive,ambient gas applied to the outside of the assembly and any first gastrapped within the void 140. The average diameter of the void 140 willthus be reduced allowing the encapsulant to begin encapsulating theoutermost solder connections 110 on all four sides of the chip 100. Thispressure differential further causes the encapsulant 130 to flow betweenthe chip and the substrate and around the connections. The amount ofpressure required to collapse the void 140 depends on the type ofencapsulant 130 used and on the desired time period to entirely collapsethe void 140. Typically, if more pressure is used, less time is requiredto collapse the void 140. The amount of pressure applied may also dependon how the pressure is applied, i.e. all at once in a virtual pressure“step” or gradually in progressively increasing pressure steps. Whileeither pressure application method may be used, using one virtualpressure step increases the rate of collapse of the void 140. By way ofexample, the applied ambient gas pressure may be approximately between10 and 1000 pounds per square inch (“psi”) and will be applied forbetween thirty minutes to several hours; although typically no harm willcome to the assembly if it is allowed to remain within the pressurizedenvironment beyond this specified time frame. Such pressure may besupplied by an autoclave device such as has been used in the aerospaceindustry; although the required size of an autoclave for the removal ofvoids within encapsulation is typically much smaller than required forthe aerospace industry. An example of a possible autoclave device is theMini-bonder autoclave manufactured by United McGill of Westerville,Ohio.

Eventually, the trapped first gas in the void 140 will begin to reach anequilibrium state with the ambient pressure applied to the outside ofthe assembly, i.e. the pressure of the first gas (P₁) will beapproximately the same as the ambient pressure (P₂). At this time, theadded pressure due to the surface tension (P_(t)) of the void 140 beginsto play a more dominant role. As the void 140 gets smaller, the surfacetension induced pressure acting on any first gas within the void 140gets larger. At some point, during a second time period, the sum of thepressures within the void 140 (P₁) will become greater than the ambientpressure (P₂) acting on the outside of the assembly. At or near thispoint, the first gas within the void 140 will slowly diffuse into theencapsulant 130. Since the gas within the void 140 is under higherpressure, during the second time period, than the ambient gas acting onthe outside of the assembly, the rate of diffusion of the first gas intothe encapsulant 130 will be greater than the rate of diffusion of theambient gas into the encapsulant 130 causing the void 140 to get smallerstill. As the diameter of the void 140 gets smaller and smaller, thepressure due to the surface tension (P_(t)) of the void 140 willprogressively grow greater and greater, up until the point where the gaswithin the void is completely diffused within the encapsulant, as shownin FIG. 1D, and the void implodes.

If the area around the flip chip device has been evacuated prior to thesealing step, there will of course be virtually no first gas within thevoid 140. This being the case, the combination of the ambient pressure(P₂) and the surface tension pressure (P_(t)) “push the encapsulant into the void 140 while aided by a “pulling effect” created by the vacuumwithin the void 140 pulling the encapsulation into the void 140. Thus,an evacuation step prior to the sealing step will cause any voids 140 tocollapse more quickly.

Typically, the next step includes applying energy to cure theencapsulant 130. The object of applying the curing energy is to fullycure (cross-link) the encapsulant 130 so that new voids will not reoccursuch as may happen when the ambient gas pressure (P₂) is releasedquickly. The type of applied energy will depend on what encapsulant isused and how that particular encapsulant cures. Examples of possiblesources of energy include heat, ultra-violet radiation, catalysis andcombination thereof. When an epoxy, such as Hysol®, is used as theencapsulant, heat is typically applied as the curing energy. Anappropriate temperature range will again depend on the type ofencapsulant 130 used and the length of time desired to fully cure theencapsulant 130. A fully cured, thermoset encapsulant 130 will maintainits structural integrity without allowing new voids to occur. It hasalso been found that the step of applying energy may occur after theambient gas pressure (P₂) is removed from the assembly without thereoccurrence of voids 140 if the ambient gas pressure (P₂) is releasedslowly enough so as to allow any dissolved gases within the encapsulant130 to come to equilibrium with the surrounding ambient gas.

In a variation of this process, a first gas within the void 140 and anambient gas may be comprised of different gases to facilitate thediffusion of the first gas into and through the encapsulant 130 at agreater rate than the rate of diffusion of the ambient gas into theencapsulant. After the step of sealing the gap between the chip 100 andthe substrate 120, the first gas surrounding the assembly may beevacuated and another ambient gas may take its place. In such anembodiment, the first gas contained within the void 140 may have smallermolecules than the ambient gas accelerates first gas' diffusion into theencapsulant 130. Choosing the correct sealed first gas will necessarilyentail choosing the correct ambient gas and vice-versa. Examples ofsealed first gases include helium, hydrogen, H₂O vapor, etc. Examples ofambient gases when compared with the first gases listed above includeargon, air, nitrogen, carbon dioxide and Krypton.

In a further variant of this process, hydrostatic pressure could be usedinstead of the isostatic pressure discussed above to compress thevoids/bubbles. Typically in such a process, inert fluids would be usedto compress the voids/bubbles.

II. Injection of an Encapsulant

As shown in FIGS. 2A-C, the methods described above in connection withencapsulating flip chip device may also be used to inject a fluidencapsulant between a first intermediate substrate 225 and a chip 200 aswell. One such embodiment where injection of encapsulation is describedis in U.S. patent application Ser. No. 08/365,699 filed Dec. 12, 1994,which is hereby incorporated by reference. In brief, as shown in part inFIGS. 2A-C, the '699 application describes a method and structure fordisposing a plurality of compliant pads 250 which provide a stand off orgap between a first sheet-like support structure 225, and a contactbearing face surface of a semiconductor chip 200. The first substrate225 may be rigid, semi-rigid or flexible. For added CTE compensation,preferably the first substrate 225 is a substantially inextensibledielectric film, such as polyimide having a thickness of between 0.5mils and 3 mils.

As described above, the adjacent compliant pads 250 provide thestructure for the gap between the chip 200 and the first substrate 225and provide and define channels therebetween for the encapsulant 230 toeventually flow; however other alternatives could be used instead, suchas cellular foam, loosely woven non-conductive strands, etc. It is onlyimportant that a gap be provided between the chip 200 and the firstsubstrate 225 while holding the two items in a substantially coplanarrelationship. The compliant pad embodiment will be described in thisexample. As described in the '669 application, the compliant pads 250are typically deposited, as by stenciling, and cured so that the chip200 and the first substrate 225 are attached to each other in asubstantially coplanar relationship. A curable liquid encapsulant 230may then be deposited completely around the perimeter of the gap betweenthe chip 200 and the first substrate 225 so as to seal the pads 250 andcreate a sealed void (void/gas bubble) 240 out of the network of thechannels therewithin, very much like the sealed assembly described inthe flip chip embodiment above. Typically, a curable, compliantthermoset resin is used as an encapsulant, such as silicone andflexiblized epoxy. Alternately, thermoplastic materials may be used ifthey are specially formulated to undergo a phase change such that theygo liquid under certain conditions and not under others, such as at atemperature which would be typically higher than the normal operatingtemperature of the resulting device.

An isostatic pressure is then applied to the assembly causing theencapsulant to flow between the chip 200 and the first substrate 225 andinto the channels within the void 240. Again, the ambient gas pressureapplied will depend on the encapsulant material selected and the amountof time desired for the process of removing the void 240; and further onwhether the pressure is applied in a virtual step or is appliedgradually. The typical applied ambient gas pressure here will beapproximately between 10 and 450 pounds per square inch (“psi”), andpreferably between about 30 and 200 psi, for a time period of betweenabout thirty minutes and several hours (the time period also depends onthe volume to be encapsulated). A virtual step of ambient gas pressureis preferred in order to increase the rapidity of the void removal.

As described above, the step of applying pressure on the assembly afterthe void 240 has been sealed will cause a pressure differential betweenthe compressive, ambient gas applied to the outside of the assembly andany first gas trapped within the void 240. The average diameter of thevoid 240 will thus be reduced allowing the encapsulant to beginencapsulating the outermost compliant pads 250 on all four sides of thechip 200.

The pressure from the ambient gas combined with the pressure from thesurface tension of the void 240 itself will cause the void to growsmaller. If there is a first gas trapped within the void, as the firstgas pressure becomes greater than the pressure due to the ambient gaspressure, the first gas will begin to diffuse into the encapsulant 230.Since any first gas within the void 240 is under higher pressure, duringthe second time period, than the ambient gas acting on the outside ofthe assembly, the rate of diffusion of the first gas into theencapsulant will be greater than the rate of diffusion of the ambientgas into the encapsulant. As the diameter of the void 240 gets smallerand smaller, the pressure due to the surface tension of the void 240will progressively grow greater and greater, up until the point wherethe void 240 is completely removed from the encapsulant, as shown inFIG. 2C.

As described in the last section, energy is typically applied to curethe encapsulant 230. The type of energy applied depends on theencapsulant 230 selected and under what conditions the selectedencapsulant 230 cures. If a silicone is used as the encapsulant, theenergy applied is typically heat. The amount of heat and the length oftime the heat is applied is again dependent upon the selectedencapsulant material and the volume of material that needs to be cured.As described above, the application of energy will typically occur priorto the termination of the step of applying pressure so that no new voidsare allowed to develop in the encapsulant by the reduction of theambient pressure. A fully cured, thermoset encapsulant 130 will maintainits structural integrity so that gas bubbles and voids do not reenterthe encapsulant. The above mentioned variations and embodiments listedin the Flip Chip Underfill Encapsulation section, above, would alsofacilitate this injection process.

Thus, in the configuration shown in FIG. 2C, the internal high stresszones in the chip package assembly are reduced by the substantialremoval of the voids and bubbles in the cured compliant layer 270. Thismethod of injection also works where the encapsulant is injected betweena first substrate and a second substrate, such as in a fan-out chippackage embodiment where at least some of the terminals 210 are disposedabove a second support structure such as a thermal spreader or ring, asdescribed more fully in the '669 application.

As described above and shown in FIGS. 2D-2G, the above injection methodsare preferably performed on more than one assembly at a time tofacilitate the mass production of finished parts. FIG. 2D shows aperspective view of a plurality of chips 200 attached to a substrate222, which includes the first substrate 225 and a sacrificial outerportion 227. An outer portion of the substrate 222 is also attached to arigid or semi-rigid frame 280 to support the typically flexiblesubstrate 222 such that it is stretched taut across the frame andfurther to better ensure the dimensional stability of the substrate.Since the combined thickness of the chips 200 and the substrate 222 isfairly thin, the thickness of the frame 280 is also typically thin sothat a plurality of such assembled frames 221 may be placed in the samepressure/temperature vessel after the encapsulant has been deposited, asdescribed above.

FIG. 2E shows a nozzle 235 for depositing the encapsulant 230 around theedges of the gap between the chips 200 and the first substrate 225 so asto seal the voids 240. A CAM/ALOT programmable dispensing machinemanufactured by Camelot Systems, Inc. in Haverhill, Mass. could be usedto perform such a task. In this configuration, the encapsulant 230 isdeposited from the chip side of the frame assembly 221. A layer ofsheet-like material 229, such as a coverlay or solder mask material, isattached to the terminal 210 side of the substrate 222 so that theencapsulant is bounded and does not escape through the bonding apertures228 (FIG. 21) such that it could contaminate the terminals and thusimpede any subsequent electrical connection of the terminals to the bondpads on the circuitized substrate 220. The frame 280 also acts as abarrier to provide a side boundary for the deposited encapsulant shouldthe need arise. However, preferably, the encapsulant 230 has a thickenough consistency such that it substantially maintains its shapethrough the surface tension of the material. Most preferably, theencapsulant 230 is deposited such that it does not flow onto the backsurface (non-contact bearing surface) of the chip 200. This allows theback surface of the chip to subsequently be connected to a heat sink orthermal spreader without an insulative material impeding the dissipationof heat from the chip 200 during thermal cycling.

After the gap has been sealed, the frame assembly 222 is placed within apressure chamber, such as the aforementioned autoclave chamber, and anisostatic pressure is applied to collapse the voids and/or gas bubbles240 thereby causing the encapsulant to engulf the supports 250 andprovide a substantially void free interposer layer, as described above.Preferably, a plurality of frames 280 are placed within a frame assemblycarrier 285 so that the frames 280 may be more easily stacked togetherand placed within the pressure chamber. In the embodiment shown in FIG.2F, the frames are placed within the carrier 285 such that the oppositeedges of the frames rest upon small shelves or projections 286 extendingfrom the side walls of the carrier 285. In another examples, the frames280 may simply be optimized such that a plurality of such frames 280 maybe stacked one on top of the other such that the individual frames donot come into contact with the substrate 222 or chips 200 on an adjacentframe 280. Energy is then applied to cure the encapsulant, as describedabove.

The fully encapsulated semiconductor chip packages within the frameassemblies 221 are next separated (or “diced”) from their respectiveframe/substrate into single packaged chips, such as that shown in FIG.2G, or the packaged chip may be diced into multi-chip modules. Thedicing operation is typically performed by laser cutting, stamping orsawing (such as with a water saw). The component in FIG. 2G has somebenefits when compared with the embodiment depicted in FIG. 2C. First,the first substrate 225 is extended to be the same size as the contact270 bearing surface of the chip 200. This allows the cured encapsulantlayer 230 (interposer layer) to have a substantially uniform thicknessat every point between the chip 200 and the first substrate 225, even atthe periphery of the package. This provides added support for the leads260 during thermal cycling of the component as well as added physicaland environmental protection for the metallurgy of the connectionbetween the leads 260 and the contacts 270.

FIG. 2H shows a side view of an alternate embodiment which is similar tothe embodiment shown in FIG. 2G. However, in FIG. 2H, the firstsubstrate 225 is juxtaposed with the contact 270 bearing surface of thechip, but is larger than the surface of the chip 200 such that ituniformly extends beyond the perimeter of the chip 200. The interposerlayer 230 correspondingly extends beyond the chip perimeter to form abumper of encapsulant material 232. This structure may be created bydicing the packaged chips a short distance beyond the periphery of thechip 200 itself. The bumper 232 further protects the chip and themetallurgy of the connection of the leads 260 to the contacts 270.

FIG. 2I shows a face surface view of the chip package described in FIGS.2G and 2H. The bonding windows 228 allow the leads 260 to be detachedfrom the sacrificial substrate 227 and be bonded to respective chipcontacts 270.

FIG. 2J shows a cross section view of a center bonded package as thevoid 240 is being diffused into the encapsulation material 230. Thepackage in this FIG. is similar to the package shown in FIGS. 2A and 2Bexcept that in FIG. 2J the die 200 is rectangular and the plurality ofcompliant pads 250 have been replaced by two compliant pads 250′ whichcreate a single center channel 240′ therebetween instead of rows andcolumns of channels 240. Also, the leads 260′ in FIG. 2J are chipcontacts which are located in a central portion of the contact bearingsurface of the chip such that the leads 260′ extend from either side ofchannel 240′ and are bonded to the contacts in somewhat of aninterleaving pattern, instead of being bonded to contacts which areperipherally positioned on the chip. The encapsulant 230 is againdeposited at the edges of the gap created by the compliant pads 250′sealing void within channel 240′. FIG. 2K shows cross section A fromFIG. 2J depicting the collapsing of the void 240′ in response toisostatic pressure and any first gas contained within the void beingdiffused into the encapsulant, as described in the above embodiments.FIG. 2L shows the same type of void 240″ collapse described above exceptthat the compliant pads 250″ define a differently shaped void 240″. Inthis Figure, the compliant pads 250″ are located in the central portionof the chip 200 and beyond the periphery of the chip 200 (typicallythere is a support structure surrounding the chip to support theperipheral pads). This arrangement allows for a so called “fan-out” or“fan-in/fan-out” semiconductor package embodiment in which the terminals210 on the substrate 225 may be located in above the chip surface orbeyond the chip's periphery or both, as described above.

FIGS. 2M-1 through 2M-7 graphically shows the above described method(discussed in relation to FIGS. 2D through 21) with respect to thecenter bonded embodiment of FIG. 2J. In this embodiment, the chip 200 isattached to the compliant pads 250′ attached to the tape 225. The tapehas a bonding window so that the leads 260′ may extend and typicallydetachably held in place thereacross. As shown in FIG. 2M-2, the leadsare detached from the substrate 225, guided and bonded to respectivechip contacts 270′ using a bonding tool 231 which typically bonds theleads to the contacts using ultrasonic or thermosonic orthermo-compression means or some combination thereof, as shown. FIG.2M-3 shows a protective sheet-like layer 229, such as a coverlay orsolder mask layer, is attached to the terminal 210 side of the substrate222 so that the encapsulant may be bounded and will not escape throughthe bonding apertures 228′ so that it does not contaminate the terminalsand impede any subsequent electrical connection of the terminals 210 tothe bond pads on the circuitized substrate.

FIG. 2M-4 shows a nozzle 235 depositing encapsulation material 230around the chip 200 on the chip/compliant pad bearing surface of thesubstrate 222 so as to seal the gap/void 240′ between the chip 200 andthe substrate 225, as described in more detail above. It is evident fromthis that the protective layer 229 is performing the function of sealingany apertures in the substrate 225, such as the bonding aperture 228′.FIG. 2M-5 shows the application of isostatic force and the resultingcollapse of the void 240′. At this point, any gas which may be withinthe void 240′ diffuses into the encapsulation material 230 at a ratewhich is faster than the rate of gas diffusion from outside the sealedarea causing the voids 240′ to reduce in size and further causing theeventual removal of the voids 240′, as shown in FIG. 2M-6. FIG. 2M-6further shows the application of solderballs 211 to the terminal sites210. FIG. 2M-7 shows a finished semiconductor package after it has beencut away (or “diced”) from the frame assembly, shown in FIG. 2D, bylaser cutting, stamping or sawing (such as with a water saw). In thisparticular embodiment, the package is diced such that a bumper 232 ofencapsulation material is left along the side edges of the chip 200,similar to the embodiment shown in FIG. 2H. In a variation, the packagemay be diced such that there is no bumper 232. In a further variation, afinal step may be added in which a thermoset or thermoplastic resinsheet-like layer is vacuum laminated onto the back of the chip 200 andaround each of the sides of the package to add further protection anddurability to the package.

FIGS. 2N-1 through 2N-7 show a further variation on the processes andstructures shown in FIG. 2M-1 through 2M-7 in which a reworkable packagestructure is disclosed. In FIG. 2N-1, a compliant layer 250′″ isattached to the substrate 225 by means of a peelable tacky surface251′″. This peelable compliant layer must be compliant enough toaccommodate the CTE mismatch problems in thermal cycling of the finisheddevice, yet have the ability to be removed (or peeled off) from thesubstrate 225 or allow the chip 200 to be removed after attachmentwithout substantial residue on the substrate or chip. Examples ofpossible materials for the peelable compliant layer 230 includestructures which have inherent releasable adhesive properties, such aspressure sensitive adhesives, and structures which may be “impregnated”so that they have such releasable adhesive properties, such as curedsilicone pads impregnated with pressure sensitive adhesives or fullycured pads of tacky silicone gel, or fully cured compliant padstructures which have releasable adhesive on the major opposingsurfaces.

As shown in FIG. 2N-2 and described above, the chip 200 is then attachedto the tacky surface 251′″ and the leads are bonded. If there is aproblem or defect with the package up to this point, the chip 200 may beremoved and reused hereby providing a cost saving to chip manufacturers.Further, if the problem is with the pad 230 and the lead bondingfunction has not occurred, the pad may be removed from the substrate 225and a new pad 250′″ may be used in its place.

Such a releasable package structure would encounter durability problemsin use of the finished package due to the possibility of the pads250′″/251′″ peeling away from either the substrate 225 or the chip 200or both unless, at some point before the package is shipped to acustomer, the peelable nature of the pad is neutralized. FIG. 2N-4 showsthe encapsulant 230 being deposited on the substrate 222 at theperiphery of the chip 200 so as to seal the area between the chip 200and the substrate 225, as explained earlier. Isostatic pressure isplaced on the assembly so as to reduce the volume of the void 240. Asexplained above, during this step, any first gas trapped within the void240 diffuses into the encapsulant until the void 240 has been completelyremoved, as shown in FIG. 2N-6. At this point, the encapsulantcompletely surrounds the peelable compliant pad 250′″ thereby providedthe needed means to bind the chip 200 to the substrate 225 prior toshipment to an end user, as shown in FIG. 2N-7.

In a variation to this process, a pressure sealed membrane 231 may beapplied or deposited in a sheet form atop the assembly, as shown inFIGS. 2N-5 and 2N-6, prior to the application of pressure thereby, ineffect, laminating the membrane to the structure (back surface of thechip 200 and exposed surface of the encapsulant 230). Such a membrane231 may be permanently sealed to the back of the chip 200 protecting thechip 200 and further binding the bumper portions 232 so that they do notde-laminate from the side edges of the chip 200 after the package hasbeen diced, as shown in FIG. 2N-7. Alternately, the membrane 231 may beremoved from the back of the die leaving the back surface of the chip200 bare to be subsequently attached to a cooling surface or heat sink.

In a still further variant of this process, hydrostatic pressure couldbe used instead of the isostatic pressure discussed above to compressthe voids/bubbles. Typically in such a process, inert fluids would beused to compress the voids/bubbles.

III. Treating an Interposer Layer

FIGS. 3A-E show a similar design to that shown in FIGS. 2A-C; however,in this embodiment, an interposer layer, such as a compliant pad 330, isdisposed between the first substrate 325 and the semiconductor chip 300.The present invention provides a method of treating the pad 330 toprovide a substantially void free layer. Typically, the compliant pad330 is comprised of a curable thermoset resin, such as silicone,flexiblized epoxy, urethane elastomer or polyimide foam. In theembodiments shown in FIGS. 3A-E, the first substrate 325 is typicallycomprised of a substantially inextensible sheet-like material, such aspolyimide, which may be substantially rigid, semi-rigid or flexible.

The pad 330 may be positioned and attached as a solid piece, as withadhesive on opposing sides thereof, or it may be stenciled or screenedonto the face surface of the chip 300 or onto the opposing surface ofthe first substrate 325. Typically, it is stenciled onto the firstsubstrate 325 prior to the leads 360 being detached from a sacrificialouter portion 327 of the first substrate and formed and bonded torespective chip contacts 370. The stenciled pad 330 is then at leastpartially cured. If a thicker compliant pad 330 is desired or if addedadhesive properties are required a chip attach layer (not shown) maythen be deposited on top of a typically fully cured pad 330, as by astenciling step. The chip attach layer may be then be left eitheruncured or may be partially cured (“B-staged”) prior to the attachmentof the chip 300. Preferably, the chip is attached using a heated colletplacement system so that the chip is relatively hot when it is pressedagainst the chip attach layer so as to minimize the number and size ofany voids (or gas bubbles) 340/340′. Typically, these voids will occurat the boundary between the different materials in the assembly, e.g.first substrate/compliant pad, compliant pad/chip or even compliantpad/chip attach layer depending upon the materials chosen for those twomaterials. Even using a hot chip placement system, gas bubbles or voids340/340′ are difficult to avoid in the uncured chip attach layer whetherfrom the entrapment of gas when the chip is attached (FIG. 3B) or whenthe pad 330 is stenciled (FIG. 3C).

After the chip 300 has been attached and the voids 340/340′ have beensealed within the package assembly, the assembly is placed underisostatic pressure for a given amount of time to remove the voids340/340′. As in the embodiments described above, the step of applying anisostatic ambient gas to the outside of the assembly is used to create apressure gradient between the lower pressure first gas/vacuum within thevoids 340/340′ and the ambient gas pressure be applied to the outside ofthe assembly. This has the effect of compressing any first gas in thevoids 340/340′ thereby reducing the volume of the void and increasingthe pressure therewith. If there is a sealed first gas within the voids340/340′, the pressure within the each void 340/340′ is the sum of thepressure from the compressed first gas and the pressure from the surfacetension of the void. As the void volume decreases, the pressure from thesurface tension of the void will increase dramatically. At some pointduring a second time period, the sum of the pressures within the void340 will become greater than the ambient pressure acting on the outsideof the assembly. Because of the greater pressure within the void inrelation to the pressure outside the assembly during second time period,the first gas within the void will begin to diffuse into the compliantpad/chip attach layer 330 faster than the ambient gas diffuses into thecompliant pad/chip attach layer 330. The greater rate of diffusion ofthe first gas combined with the increased pressure of the first gas willcause the first gas to completely diffuse into the compliant pad/chipattach layer 330 such that the bubble has been removed.

As described in the above sections, the pressure which is needed toremove the voids will depend on the materials used as the compliant padand the hip attach layer and will also depend on the time allotted fortheir removal and the total volume of the voids to be removed. Anexample of a suitable pressure/time range where both the compliant padand the chip attach layers are comprised of silicone resin includesbetween about 10 and 1000 psi for anywhere over approximately one hour.The assembly may be kept under pressure for more than the specifiedamount of time, without harm coming to it.

Any first gas sealed within the void 340 may be comprised of the samegas as the ambient gas; however, the first gas and the ambient gas mayalso be different in order to facilitate a greater rate of diffusion ofthe sealed first gas. After the step of sealing the first gas byattaching the chip to the compliant pad/chip attach layer 330, the firstgas present on the outside of the assembly may be evacuated and replacedwith a different pressurized ambient gas. Desirably, the second ambientgas should have an inherent rate of diffusivity with respect to the chipattach layer material that is less than the rate of diffusivity of thefirst gas into the same chip attach material. If the chip attachmaterial is a curable thermoset resin material, suitable examples ofpossible first gases include: helium, hydrogen, H₂O vapor, methane andfluorinated hydrocarbons. Examples of suitable second ambient gases insuch a situation include: argon, air, nitrogen, and carbon dioxide.

Typically, after the voids 340/340′ have been removed from the compliantpad/chip attach layer 330, the assembly is heated in order to cure (orfully cross link) the chip attach layer to ensure that the voids340/340′ do not return prior to the removal of the ambient gas pressurebeing applied the assembly. As described above, other types of energymay be used to cause the chip attach layer to cure depending upon whatthe material is comprised.

As shown in FIG. 3D, after the voids 340/340′ have been removed and thecompliant pad is cured, the leads 350 are typically detached from thesacrificial outer portion 327 and coupled to respective chip contacts370. These leads are then encapsulated, as shown in FIG. 3E, to protectthem from corrosion and to support them during thermal cycling of thefinished chip package assembly. If voids are present in the leadencapsulant material, the voids may be removed by using theaforementioned methods. The encapsulation of the leads may also beperformed using the same techniques described in the “back-sideencapsulation” embodiment of FIGS. 2D-2H if the user desires tosimultaneously produce a plurality of components.

In a further variant of this process, hydrostatic pressure could be usedinstead of the isostatic pressure discussed above to compress thevoids/bubbles. Typically in such a process, inert fluids would be usedto compress the voids/bubbles.

The method of the present invention has thus far been described withrespect to individual semiconductor chips. However, it is contemplatedthat the method of encapsulation may be employed with a plurality ofchips simultaneously provided on a semiconductor wafer. As illustratedin FIG. 4A, chips 428 may be provided in the form of a semiconductorwafer 430 incorporating a plurality of such chips, all of the samedesign or of differing designs. Individual interposers 442 may bepositioned on the individual chips 428 constituting wafer 430, and theinterposers may be assembled to the chips as discussed above. In thisoperation, the contacts on each chip 428 are secured to the leads andterminals of each interposer. The encapsulation of the interposer 442and chips 428 can be accomplished in accordance with the methods of thepresent invention as thus far described with respect to individualsemiconductor chips. In addition, where an interposer 442 is omitted,encapsulation of the plurality of chips 428 on the semiconductor wafercan be achieved in accordance with the previously described process, forexample, flip chip underfill encapsulation and injection of theencapsulant.

After the interposers 442 and chips 428 are encapsulated, the individualchips are separated from the wafer 430 and from one another, as bycutting the wafer using conventional wafer severing or “dicing”equipment commonly utilized to sever individual chips withoutinterposers. This procedure yields a plurality of encapsulated chips andinterposer subassemblies, each of which may be secured to an individualsubstrate.

As illustrated in FIG. 4B, a wafer 530 incorporating a plurality ofinterposers 542. Again, the contacts on each chip are secured to theterminals and leads of one individual interposer overlying theparticular chip. The encapsulation of the chip and interposer may beaccomplished in using any of the methods as thus far described. Thewafer 530 and the sheet 535 are severed after this operation, anddesirably after encapsulating, so as to provide individual subassemblieseach including a chip and an interposer.

Having fully described several embodiments of the present invention, itwill be apparent to those of ordinary skill in the art that numerousalternatives and equivalents exist which do not depart from theinvention set forth above. It is therefore to be understood that thepresent invention is not to be limited by the foregoing description, butonly by the appended claims.

What is claimed is:
 1. A method of providing a substantially void freeunderfill for a flip chip assembly, comprising: electrically connectinga plurality of contact pads on a surface of a semiconductor chip tocorresponding bond pads on a circuitized substrate such that theconnections create a gap between the chip and the substrate, wherein thesubstrate is rigid; sealing the gap between the chip and the substratewith a fluid, curable encapsulant so that there is a void therebetween;applying pressure to the assembly causing the encapsulant to flow intothe gap and around the connections; and applying energy to the assemblyin order to cure the encapsulant.
 2. The method as claimed in claim 1,wherein the pressure applying step is conducted for a time period thatis between 30 minutes to several hours.
 3. The method as claimed inclaim 1, wherein the pressure applying includes gradually increasing theapplied pressure.
 4. A method of injecting an encapsulant between a facesurface of a semiconductor chip and a juxtaposed substrate, comprising:providing a gap between the face surface of the chip and the substrate,wherein the substrate is rigid; sealing each edge of the gap with acurable fluid encapsulant so that there is a void between the chip andthe substrate; applying pressure to cause the encapsulant to flowbetween the chip and the substrate; and applying energy to cure theencapsulant.
 5. The method as claimed in claim 4, wherein the pressureapplying step is conducted for a time period that is between 30 minutesto several hours.
 6. The method as claimed in claim 4, wherein thepressure applying includes gradually increasing the applied pressure. 7.The method as claimed in claim 4, wherein the applied pressure isbetween about 10 and 450 pounds per square inch.
 8. The method asclaimed in claim 4, wherein the applied pressure is between about 30 and200 pounds per square inch.
 9. The method as claimed in claim 4, whereinthe gap providing step includes: providing compliant pads on thesubstrate, wherein each of the compliant pads includes a peelable tackysurface to which the face surface of the chip is releasably attached.10. A method of providing a substantially void free underfill for asemiconductor wafer having a plurality of flip chip assemblies,comprising: electrically connecting a plurality of contact pads of flipchip assemblies disposed on a surface of a semiconductor wafer tocorresponding bond pads on a circuitized substrate such that theconnections create a gap between the flip chip assemblies and thesubstrate, wherein the substrate is rigid; sealing the gap between theflip chip assemblies and the substrate with a fluid, curable encapsulantso that there is a void therebetween; applying pressure to cause theencapsulant to flow into the gap and around the connections; andapplying energy to cure the encapsulant.
 11. The method as claimed inclaim 10, wherein the pressure applying step is conducted for a timeperiod that is between 30 minutes to several hours.
 12. The method asclaimed in claim 10, wherein the pressure applying includes graduallyincreasing the applied pressure.
 13. A method of injecting anencapsulant between a face surface of a semiconductor wafer and ajuxtaposed substrate, comprising: providing a gap between the facesurface of the wafer and the substrate, wherein the substrate is rigid;sealing each edge of the gap with a curable fluid encapsulant so thatthere is a void between the wafer and the substrate; applying pressureto cause the encapsulant to flow between the wafer and the substrate;and applying energy to cure the encapsulant.
 14. The method as claimedin claim 13, wherein the pressure applying step is conducted for a timeperiod that is between 30 minutes to several hours.
 15. The method asclaimed in claim 13, wherein the pressure applying includes graduallyincreasing the applied pressure.
 16. The method as claimed in claim 13,wherein the applied pressure is between about 10 and 450 pounds persquare inch.
 17. The method as claimed in claim 13, wherein the appliedpressure is between about 30 and 200 pounds per square inch.
 18. Themethod as claimed in claim 13, wherein the gap providing step includes:providing compliant pads on the substrate so as to provide the gapbetween the face surface of the wafer and the substrate, wherein each ofthe compliant pads includes a peelable tacky surface to which the facesurface of the wafer is releasably attached.